High molecular weight polycarbonate is a valuable engineering resin useful for producing many objects, especially clear sheeting, compact recording discs and housings for electronic equipment. There are a number of ways this resin can be produced. The most common industrial method is the interfacial polymerization method in which bisphenol A and phosgene are reacted in a heterogeneous mixture of water and methylene chloride. Although this process produces the desired high molecular weight polymer, there are disadvantages associated with it. Phosgene is extremely toxic and hence results in safety concerns. In addition the use of methylene chloride raises environmental concerns. Finally, the polymer produced by this method contains residues of sodium chloride, which are produced by neutralization of sodium hydroxide used to dissolve bisphenol A in water. This impurity is undesirable in some applications and is difficult to remove.
A second method used to produce polycarbonate is the melt polymerization of bisphenol A and diphenyl carbonate. This process requires the removal of the condensation by-product from the viscous polymer melt. The high temperatures required to achieve low viscosity can lead to degradation of the polycarbonate polymer.
A final method known for producing high molecular weight polycarbonate is solid state polymerization. In this process, a low or moderate molecular weight polymer is produced and isolated as a solid material such as chips, particles, granules, or powders. Particles of controlled size and shape are most desirable. The polymerization of this solid material is accomplished by heating it to a temperature below its melting temperature with a heated inert gas. The solid state polymerization is thus carried out at lower temperature, which reduces the degradation problem. Before this final step of the solid state polymerization is carried out, the starting materials must be crystallized. For polycarbonate, this step is known to be very difficult because of the slow crystallization rate of polycarbonate. Although technologies for crystallization of polycarbonate have been described, all of these technologies have serious drawbacks associated with them.
The difficulties in crystallizing polycarbonate prior to solid state polymerization are related to the slow development of crystallinity in this polymer. The time required to obtain the maximum level of crystallinity in polycarbonate is much longer than for other polymers. The crystallization rate found for polycarbonate oligomer is greater than that of high molecular weight polycarbonate; but it is still very low compared to other polymers, such as polyethylene terephthalate, of similar molecular weight; i.e., it exhibits much longer crystallization times.
The second factor limiting crystallization rate is nucleation. It is generally known that the rate of growth of crystallization can be accelerated in polymers by the addition of a nucleating agent. Examples of commonly used nucleating agents include inorganic oxide materials such as talc, or organic salts such as sodium benzoate. These materials suffer from a common weakness in that they require the addition of a foreign substance, essentially an impurity, to the polycarbonate resin to be produced. In many applications, this can adversely affect the end use properties of the resin.
European Patent No. 0 864 597 discloses a process for the solid state polymerization of polycarbonate oligomer under an atmosphere of a swelling solvent gas or under a stream of a poor solvent gas. The process is applied to either amorphous oligomer particles or powders or to semicrystalline particles or powders. The process does not include a separate crystallization step and hence does not allow one to control the conditions under which crystallization occurs. The swelling solvent gas or poor solvent gas is present throughout the process along with a second inert gas. Since this mixed gas stream will also contain condensation by-products that must be removed during the solid state polymerization, the required constant presence of swelling or poor solvent gas complicates the gas handling requirements of this process, especially if the gas is recycled. Suitable swelling solvents listed include aromatic hydrocarbons, e.g., benzene and substituted benzenes; ethers, e.g., tetrahydrofuran and dioxane; and ketones, e.g., methyl ethyl ketone. Suitable poor solvent gases listed include cyclic hydrocarbons, straight chain or branched saturated hydrocarbons, and unsaturated hydrocarbons.
U.S. Pat. No. 5,191,001 discloses a process for the production of polycarbonate by solid state polymerization of an intimate mixture of oligomeric polycarbonates. The oligomers to be used in this process have a particular endgroup composition. Although crystallization is a required step for this process, the authors do not disclose any particular crystallization technology. A number of general schemes of possible applicability to many polymers are included. The only crystallization method applied is the well-known solution procedure where a semicrystalline powder is prepared by solvent removal from a solution of the oligomers in methylene chloride.
U.S. Pat. No. 5,717,056 discloses a method for preparing a polycarbonate comprising the steps of (a) converting a precursor polycarbonate to an enhanced crystallinity precursor polycarbonate, and (b) polymerizing in the solid state. Converting the precursor polycarbonate to an enhanced crystallinity precursor polycarbonate entails contact at above 110° C. with a basic compound. Specific basic compounds listed include alkali metal hydroxides, tetraalkylammonium hydroxides, tetraalkylammonium carboxylates, tetraalkylphosphonium hydroxides, and tetraalkylphosphonium hydroxides. The preferred basic compounds are tetramethylammonium maleate and tetraethylammonium hydroxide. The procedure described to produce this enhanced crystallinity precursor polycarbonate involves contact of polycarbonate particles with a solution containing this basic compound followed by a thermal treatment.
European Patent No. 0 848 030 discloses a process for crystallizing a polycarbonate prepolymer comprising dissolving it in a solvent at elevated temperatures, then cooling the solution to effect crystallization. Preferred solvents are aromatic compounds which form solutions of a concentration of 20-90% polycarbonate. The crystalline product produced is then shaped into the form desired for solid state polymerization. This shape is then dried to volatilize the solvent. This process requires many steps to produce the desired crystallized product.
Japanese Patent Heisei 93 178979 discloses a process for the manufacture of aromatic polycarbonate by solid state polymerizing crystalline polycarbonate prepolymer characterized in that intermediate polymer that has been solid state polymerized is treated with a crystallization solvent and then subjected again to solid state polymerization.
It is well known that polycarbonate can be crystallized by exposure to solvents such as acetone. U.S. Pat. No. 5,214,073 discloses a method for preparing a porous crystallized polycarbonate oligomer or prepolymer. In one process described an amorphous polycarbonate oligomer is slurried with acetone to produce the crystallized polycarbonate oligomer. The large amorphous particles that are charged to the acetone bath break up into a very fine powder during the crystallization process. A second process described consists of the melt extrusion of the prepolymer melt into a stirred volume of acetone. This also produces a very fine crystallized powder. Both powders are dried before being subjected to the solid state polymerization. A very fine powder is often not desirable in solid state polymerization because of difficulties associated with material handling.
U.S. Pat. No. 6,534,623 discloses a process for the preparation of crystalline polycarbonate oligomer compositions from amorphous polycarbonate oligomer compositions comprising the steps of preparing a mixture of the amorphous polycarbonate with a fugitive crystallization enhancing agent, such as n-butyl stearate, and/or a high melting particulate polymeric nucleating agent, such as crystallized polycarbonate oligomer; forming this mixture into a shape desired; and crystallizing this mixture at a temperature above its glass transition temperature. The fugitive crystallization enhancing agent could present a cycle time problem in practice if frequent shut downs for cleaning were needed. The crystallized polycarbonate oligomer agent was prepared by heating in a vacuum oven, followed by solid state polymerization for 24 hours.
Recently, Hu and Lesser crystallized polycarbonate and a polycarbonate/organo-modified montmorillonite clay nanocomposite in the presence of supercritical carbon dioxide (Xianbo Hu and Alan Lesser, “Enhanced crystallization of polycarbonate by nano-scale clays in supercritical carbon dioxides,” “Abstracts of Papers,” 226th ACS National Meeting, New York, N.Y., United States, Sep. 7-11, 2003 (2003), PMSE-388. Publisher: American Chemical Society, Washington, D.C.). While the presence of the montmorillonite enhanced the degree of crystallization of polycarbonate in the presence of supercritical carbon dioxide, the crystallization behavior of polycarbonate was unchanged by the presence of the montmorillonite clay in the absence of supercritical carbon dioxide.
Nanocomposites are polymers reinforced with nanometer sized particles, i.e., particles with a dimension on the order of 1 to several hundred nanometers. These materials can be used in structural, semistructural, high heat underhood, and Class A automotive components, among others, offering a variety of desirable properties including: low coefficient of thermal expansion, high heat deflection temperatures, lightweight, improved scratch resistance, and potential application in automotive Class A surfaces. Polycarbonate/clay nanocomposites have typically been prepared by melt compounding either sodium cloisite or organically modified montmorillonite (OMMT) into the polycarbonate. For example, Paul et al. made polycarbonate/clay nanocomposites using a twin screw extruder to melt compound polycarbonate resin and sodium montmorillonite clay that had been organically modified by cation exchange with a variety of amine salts (P. J. Yoon, D. L. Hunter, D. R. Paul, Polymer 44 (2003) 5323-5339 and 5341-5354). Unfortunately, the high temperature needed for the melt compounding can result in degradation of both the organic species with which the clay was modified, resulting in discoloration, and the polycarbonate itself, lowering its molecular weight. The long residence times needed to achieve better clay dispersion resulted in increased color.
Vaia et al. (X. Huang, S. Lewis, W. J. Brittain, and R. A. Vaia. Macromolecules 33 (2000) 2000-2004) prepared partially exfoliated polycarbonate nanocomposite by first mixing cyclic carbonate oligomers with ditallow dimethyl-exchanged montmorillonite in a Brabender mixer at 180° C. for one hour, which partially exfoliated the montmorillonite, followed by ring-opening polymerization of the cyclic carbonate oligomers, preserving the partial exfoliation. However, an analogous experiment using linear polycarbonate instead of the cyclic oligomers produced an intercalated structure (alternating layers of polymer and silicate with a repeat distance of only a few nanometers), rather than the high dispersion of an exfoliated structure. Conventional melt or solution processing of the ditallow dimethyl-exchanged montmorillonite with either cyclic carbonate oligomers or linear polycarbonate similarly produced intercalated structures.
There remains a need for an efficient, effective, environmentally benign and economical process for the crystallization of polycarbonate.